Prior art processing to make dihydrocarvone isomer by this rearrangement delineates the utilization of mixed cis- and trans-limonene oxides to produce a mixture of cis- and trans-dihydrocarvones. A most economic starting material is citrus (d)-limonene to make the starting epoxides. The resulting mixed product, that is cis- and trans-dihydrocarvones, is of value in formulating flavors and perfumes.
The individual dihydrocarvone isomers, that is cis-dihydrocarvone and trans-dihydrocarvone, (if provided selectively say, from citrus limonene) would be of substantially more value, but such dihydrocarvone isomers cannot be separated in practical fashion by fractional distillation. Such cis-isomer is characterized by what often is called a musty and woody odor, the trans-isomer by an odor of spearmint. In admixture one can detract from the other for various organoleptic use. Accordingly, it has been proposed to synthesize the pure dihydrocarvone isomer by other routes which are more expensive, or to separate them by chromatographic processing, also an expensive procedure.
Advantages of the instant invention over prior proposals include the ability to produce reasonably economically a substantially pure cis- or trans-dihydrocarvone isomer based on limonene such as abundant citrus limonene.
The instant invention is based on two surprising discoveries that go counter to general experience in the art of heat treating unsaturated terpenoids in the presence of acid. The first such discovery is that equilibration of a substantially pure dihydrocarvone isomer can be substantially precluded while generating such isomer by the instant acid-catalyzed rearrangement process. The other discovery is that the cis-limonene oxide in a mixture of cis- and trans-limonene oxides can be transformed substantially selectively and discretely into trans-dihydrocarvone. This, of course, means that the remaining trans-limonene oxide can be further processed to make substantially pure cis-dihydrocarvone by implementing the first discovery.